Sphingolipids are ubiquitous constituents of membrane lipids in mammalian cells and play various important roles in cell growth, differentiation, and apoptosis. Further, sphingolipids are involved in membrane trafficking and intracellular signaling as a factor requiring for the formation of membrane micro domains so called lipid rafts.
The biosynthesis process of sphingolipids in mammalian cells is supposed as follows. The first step is the condensation reaction of L-serine with palmitoyl CoA, said reaction is catalyzed by serine palmitoyl transferase and generates 3-ketodihydrosphingosine. Then, the obtained 3-ketodihydrosphingosine is reduced to dihydrosphingosine. Further, the obtained dihydrosphingosine undergoes N-acylation followed by desaturation to generate ceramide (Cer). These reactions to produce Cer occur at the cytosolic surface of the endoplasmic reticulum (ER). Then, Cer is delivered to the lumenal side of the Golgi apparatus and converted to sphingomyelin (SM) by SM synthase catalyzing transfer of phosphocholine from phosphatidylcholine (PC) to Cer. Cer is also converted to glucosylceramide (GlcCer) by GlcCer synthase catalyzing the transfer of glucose from uridine 5′-diphosphate (UDP)-glucose to Cer. The outline of the biosynthetic processes to SM or to GlcCer can be illustrated by the following schema. 
Inhibitors of specific steps of various processes (reaction, transferring or others) in de novo synthesis of sphingolipids should be useful means to investigate the metabolism and functions of sphingolipids in cultured cells and in living animals.
However, no specific inhibitor of SM synthesis in mammalian cells has been found so far. In addition, no drug that selectively inhibits intracellular trafficking of sphingolipids has been discovered.
Meanwhile, in Documents 1 and 2, the inventors of the present invention have analysed the wild-type Chinese hamster ovary (CHO) cell line and its mutant cell line, LY-A, defective in the pathway of Cer transport from the ER to the site of Golgi apparatus for the site of SM synthesis. These analyses have shown that the main pathway for transport of Cer from the ER to the site of SM synthesis is ATP-dependent. On the other hand, the access of Cer to the site of GlcCer synthesis is ATP-independent. These results have also shown that inhibition of transport of Cer from the ER to the site of SM synthesis causes inhibition of SM synthesis. However, no drug that selectively inhibits the intracellular trafficking of Cer is referred in Documents 1 and 2.
Therefore, the subject of the present invention is to provide a novel inhibitor which is selective to SM synthesis. Aiming to solve said subject, the inventors of the present invention have carried out various experiments using the compounds represented by the general formula A, and found out that there is a stereoisomer which inhibits selectively the synthesis of SM. And the subject of the present invention is accomplished.